Reaction turbines have been used for centuries to convert naturally occurring phenomena, such as wind, waves, and other fluid flows, into useful power. While traditional, propeller-type turbines (e.g., a windmill) must be oriented to face the direction of the fluid flow, advances in turbine technology have led to the advent of unidirectional turbines that will rotate in the same direction for many different directions of the fluid flow without changing the orientation of the turbine. Thus, unidirectional turbines are particularly suited for use where a naturally occurring fluid flow is frequently changing directions, such as tidal estuaries or shifting winds. Over time, four basic types of unidirectional turbines have evolved—the Wells turbine, the McCormick turbine, the Darrieus turbine, and the Gorlov turbine.
Wells Turbine
The Wells turbine is a propeller-type turbine that includes a plurality of blades extending radially from a central rotating shaft. By contrast to traditional propeller-type turbines, the blades of the Wells turbine are positioned with their faces perpendicular to the plane of the fluid flow (pitch=0° or pitch=90°) instead of angled into the plane of the fluid flow (0°>pitch<90°). Accordingly, the blades of the Wells turbine can be arranged either with the blades parallel to the rotating shaft (pitch=0°) so that the blades rotate in a plane parallel to the direction of the fluid flow or with the blades perpendicular to the rotating shaft (pitch=90°) so that the blades rotate in a plane perpendicular to the direction of the fluid flow.
By further contrast to traditional propeller-type turbines, the blades of the Wells turbine utilize a symmetrical foil cross-section that allows the blades to maintain the same direction of rotation as long as the fluid flow is perpendicular to the faces of the blades. Thus, when the blades are arranged perpendicular to the rotating shaft (pitch=90°), they will rotate in the same direction for the fluid flow approaching parallel to the shaft, either from front-to-back or back-to-front (i.e., unidirectional rotation in bidirectional flow). And, when the blades are arranged parallel to the rotating shaft (pitch=0°), they will rotate in the same direction for fluid flow coming from any direction perpendicular to the rotating shaft, sweeping 360° around the rotating shaft (i.e., unidirectional rotation in omnidirectional flow). Accordingly, Wells turbines are typically mounted within channels that direct the fluid flow perpendicular to the faces of the blades, particularly with the former blade arrangement.
Although the symmetrical blades of the Wells turbine will maintain the same direction of rotation in the presence of a bi-direction fluid flow, that symmetry results in a larger drag coefficient than traditional propeller-type turbines. That higher drag coefficient results in high noise and relatively low efficiency. Moreover, the effective surface area of the symmetrical blades of the Wells turbine is limited to the outer tips where the linear velocity is greatest, preventing the blades from capturing a substantial amount of the available energy in the fluid flow closer to the rotating shaft. In addition, the Wells turbine has no of self-starting capability and must instead use a motor to start the turbine, thereby consuming energy before the turbine can be used to generate energy.
McCormick Turbine
The McCormick turbine was developed in response to the noise and efficiency problems associated with the Wells turbine. By contrast to the Wells turbine, the McCormick turbine includes a plurality of blades extending radially from a central rotating shaft utilizing an asymmetrical, substantially V-shaped, cross-section. Those rotating blades, or rotors, are mounted concentrically between a two sets of fixed blades, or stators, wherein the first set of fixed blades is open to fluid flowing from one direction and the second set of fixed blades is open to fluid flowing in an opposite direction.
The two sets of fixed blades are each positioned to direct the fluid flow into the rotating blades so that the rotating blades rotate in a plane perpendicular to the direction of fluid flow. Each set of fixed blades is configured with an opposite pitch from the set of blades on the opposing side of the rotating blades so that the rotating blades will rotate in the same direction regardless of which side of the rotating blades the fluid flow enters the turbine. Accordingly, McCormick turbines are typically mounted within channels that direct fluid flow perpendicular to the plane of rotation. Because the McCormick turbine will rotate in the same direction in the presence of bi-directional flow (i.e., unidirectional rotation in bi-directional flow), it is particularly suited for harnessing power from natural phenomena that regularly oscillate back and forth, such as wave energy.
Arrangement of the blades of the McCormick turbine allow it to be more efficient and have a smaller diameter than the Wells turbine. Thus, the McCormick turbines are quieter than the Wells turbine. That smaller diameter, however, significantly reduces the rotational speed of the McCormick turbine. The generator gearing required to compensate for that lower rotational speed is complex and expensive to manufacture.
Darrieus Turbine
The Darrieus turbine includes a plurality of blades running lengthwise along a central, vertical rotating shaft. The blades may be rectilinear, parallel to the rotating shaft, and attached to the rotating shaft at their distal ends by struts extending radially from the rotating shaft (i.e., a giromill). Or, the blades may be curved in a semicircular shape extending away from the rotating shaft with their distal ends attached at or on the rotating shaft (i.e., a troposkien turbine). The blades of the Darrieus turbine have a symmetrical foil cross-section and are disposed at a distance away from the rotating shaft so that the fluid flow can pass between the blades and the rotating shaft. That configuration allows the blades to rotate in the same direction for the fluid flow coming from any direction perpendicular to the rotating shaft, sweeping 360° around the rotating shaft (i.e., unidirectional rotation in omnidirectional flow).
In addition, because the fluid flow is allowed to pass between the blades and the rotating shaft of the Darrieus turbine, the oncoming fluid flow from the movement of the blades will create a varying force that will be vectorially added to the force created by the fluid flow moving perpendicular to the blade. The resulting added force will always point obliquely forward along the line-of-action of each blade, creating a positive torque on the rotating shaft and further helping it to rotate in the direction it is already traveling. Thus, a Darrieus turbine can have a rotational speed that is unrelated to the speed of the fluid flow and is usually many times faster.
Although the blades of the Darrieus turbine allow it to rotate at a rate higher than the fluid flow, disposing the blades away from the rotating shaft causes each blade to generate a maximum amount of torque at two different points in rotation—at the front and back of the turbine—due to the changing accelerations of the blades as they pass between high and low pressure zones in the fluid. Those changes in torque can cause strong pulsations that lower the efficiency of the turbine. Moreover, those pulsations can result in catastrophic failure of the turbine at certain wind speeds. Accordingly, Darrieus turbines typically require complicated guy wires systems to stabilize the blades and mechanical brakes or other speed control devices to keep them operating at safe speeds. Darrieus turbines also have no of self-starting capability.
Gorlov Turbine
The Gorlov turbine was developed in response to the vibration and efficiency problems associated with the Darrieus turbine. The Gorlov turbine modifies the Darrieus turbine by arranging the symmetrical foil blades in a helical configuration around a central, vertical rotating shaft rather than running them lengthwise along the rotating shaft. Thus, the blades of the Gorlov turbine are in the shape of a helix running diagonally around a cylinder. By running diagonally with respect to the rotating shaft as they curve around the shaft instead of running in line with the shaft as in the Darrieus turbine, at least a portion of each blade of the Gorlov turbine is always positioned perpendicular to the fluid flow, which provides a more continuous speed of rotation without the accelerations and decelerations caused by the blades passing between high and low pressure zones in the fluid. Moreover, each point on the blade moves in a circular path around the rotating shaft at approximately the same distance from the rotating shaft, which provides a more continuous torque curve. Accordingly, the blades of the Gorlov turbine generate less vibration and noise than a Darrieus turbine.
Although the Gorlov turbine generates a smoother torque curve than the Darrieus turbine, its helical blade shapes are complex compound curves that require the use of sophisticated manufacturing techniques, such as numerically-controlled laser cutting and composite molding and casting. Those techniques can be prohibitively expensive. Accordingly, at least one designer, Igor Palley, has attempted to approximate the helical blade design of the Gorlov patent using conventional manufacturing techniques. Palley approximates a helical blade by making several different, discrete straight sections twisted about their own axes and joining those sections in a curved manner. The more twisted, straight sections that are formed and joined in a curved manner, the closer the blade formed by those sections approximates the helical blade of the Gorlov turbine.
Despite simplifying the shape of the Gorlov turbine blades to some degree, the manufacture of Palley's approximated helical blades can also be prohibitively expensive. In addition to requiring that a large number of different, discrete straight sections be individually twisted for each blade, the offset of each section required to approximate a helix requires large computational facilities and a significant amount of skill in manufacturing and assembly. Accordingly, there remains a need for a quiet, efficient, unidirectional turbine that can operate at high speeds with minimal vibrations and that is easy and inexpensive to maintain and manufacture.